Ring laser gyroscopes have one or more sensitive axes and for each axis there is provided a gas-filled cavity or ring disposed in a plane at right angles to the associated axis, each cavity comprising three or more linear, and usually equal, limbs. Around each cavity propagate two beams of light travelling in opposite directions and directed about a closed loop, by three or more mirrors located at the respective cavity corners, and regeneratively amplified at frequencies for which the path length equals an integral number of wavelengths. Amplification is achieved through a gas discharge within the cavity, at least one anode and one cathode being provided in each cavity.
The corner mirrors of laser gyroscope cavities are very expensive components since they have to have excellent optical characteristics. A single-axis ring laser gyroscope having a three corner cavity employs three mirrors, a two-axis, three-corner cavity ring laser gyroscope employs six mirrors and a three axis, three-corner cavity ring laser gyroscope employs nine mirrors and in each case, the mirrors constitute a significant proportion of the cost of the gyroscope.
Thus a gyroscope has been developed in which a reduced number of mirrors are used. Such an instrument is described in our U.K. Pat. No. 2,076,213 and can be defined as a ring laser gyroscope having a plurality of sensitive axes and hence a plurality of cavities, the cavities lying in different planes at right angles to the respective sensitive axes and each cavity having three or more corners arranged so that at least one corner coincides with a corner of another cavity, whereby the cavities are interconnected, the gyroscope further comprising a plurality of mirrors disposed respectively at the coinciding corners and the remaining corners of the cavities, whereby the number of mirrors at the coinciding cavity corners and the coinciding cavities being oriented such that the normal to each of these mirrors lies in the plane of each cavity it serves and bisects the associated corners of those cavities.
The present invention relates specifically to such an instrument.
As the individual cavities are interconnected, a common laser medium is used and hence the beams of the cavities all operate at the same nominal frequency. As in ring laser gyroscopes in general, the beams in each cavity are prone to back scatter from the mirror surfaces. This is a mutual scattering of energy which causes inter-action of the counter rotating beams within a cavity.
In multiple axis gyroscopes of the type described, there may be cross coupling of energy not only between the counter-rotating beams within an individual cavity but also between the beams of the several cavities as a result of the shared mirrors. This may cause gyroscope errors which are more severe than those occurring in single cavity gyroscopes because the cross coupling of energy between the beams of the different cavities is larger than the back scattering of light between the counter-rotating beams of a single cavity. Moreover, this has a knock-on effect since all the beams of the several cavities are affected by the beams of any one cavity.